Throxine in vivo has been found to increase amino acid transport into brain slices from suckling rats, with an action on the A system. Work will continue to further characterize the different amino acid transport systems of brain to see how they change with development over the ages 1 to 20 days: which systems are altered by thyroxine, and how the thyroxine effect changes with age. In particular, transport by systems L and ASC will be studied, using amino acids that use these systems extensively (e.g., leucine, alanine). We will investigate further to determine why elevated plasma insulin levels of donor rats decrease the ability of insulin to stimulate transport of the model amino acid, alpha-aminoisobutyric acid (AIB), into diaphragms in vitro. Diaphragms will be exposed to insulin under differrent conditions (varying the age, time of exposure, and insulin level both in vivo and in vitro). We will also determine if the effect can be partly due to glucagon and cyclic nucleotides, agents with some actions antagonistic to those of insulin. For example, quantitative and qualitative effects of these compounds will be measured on AIB transport both at normal and elevated insulin levels.